To provide lithium ion batteries, lithium ion capacitors, electric double-layer capacitors, etc. with increased energy densities, current collectors preferably have penetrating pores to reduce positive electrode potential. Aluminum foils are widely used for current collectors, and penetrating pores are formed by various methods.
For example, JP 2011-74468 A discloses a method for producing a high-strength, porous aluminum foil by simultaneously drawing and bending an aluminum foil having large numbers of penetrating pores. The penetrating pores having inner diameters of 0.2-5 μm are formed by subjecting the aluminum foil to DC etching in an electrolyte comprising hydrochloric acid as a main component to form etched pits, and adjusting the diameters of the etched pits by chemical etching. However, because the etched pits have small inner diameters, a sufficient amount of an active material does not enter the penetrating pores, failing to obtain a sufficiently high energy density.
JP 2011-165637 A discloses a method for producing a positive current collector, which is provided with a positive active material layer to form a positive electrode of a lithium ion battery, the positive current collector being constituted by an aluminum alloy foil provided with pluralities of pit-like pores on the surface on which the positive active material layer is to be formed, the pores having an average pore diameter of 1.0-5 μm, and a ratio of average pore diameter/average pore depth being 1.0 or less, the method comprising the steps of subjecting the aluminum alloy foil to DC electrolytic etching, and treating the etched aluminum alloy foil with an aqueous organic phosphoric acid solution. However, because pit-like pores formed by DC electrolytic etching have as small an average pore diameter as 5 μm or less, a sufficient amount of an active material does not enter the pit-like pores.
JP 2012-186142 A discloses a method for producing an electrode for electrochemical devices, which is constituted by pluralities of laminated porous aluminum sheets filled with an active material, comprising the steps of filling each porous aluminum sheet with the active material, compressing it to provide a thin, porous aluminum sheet, and then laminating pluralities of active-material-filled, thin, porous aluminum sheets.
The porous aluminum sheet is produced, for example, by forming a eutectic alloy coating at a temperature equal to or lower than the melting point of A1 on a foamed resin having a three-dimensional network structure by a plating method, a vapor deposition method, a sputtering method, CVD method, etc., impregnating the foamed resin with a paste comprising A1 powder, a binder and an organic solvent as main components, and then heat-treating it in a non-oxidizing atmosphere at a temperature of 550-750° C. However, this porous aluminum sheet is disadvantageous not only in a complicated production method, but also in poor mechanical strength because of the three-dimensional network structure.
In view of the above circumstances, a porous metal foil such as a porous aluminum foil, etc. having sufficient fine pores to hold an active material, as well as high mechanical strength, which is suitably usable in lithium ion batteries, lithium ion capacitors, electric double-layer capacitors, etc., is desired.